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foundation walls. We double-checked to be sure that the inlet and outlet to the plenums were correct, then we snapped chalklines on the surface of the insulation to outline the blockbed, and just set the blocks in place (middle photo). The core holes of concrete blocks are actual- ly tapered slightly so that they will fall off the mold when they are cast. There is some question about which direction that taper should face in the blockbed since a slightly different air turbulence pattern will result for each option. Since there is no definite answer about which way is best, we solved the problem by paying no attention at all to how we set them and ended up with a random mix. We formed the sides of the supply and return plenums with a single row of blocks (bottom photo). Since this blockbed was built with 8-in. by 8-in. by 16-in. common blocks, we simply set the blocks with the cores facing up. This row of blocks also provides a ledge to support the cover we placed over the plenum. Corrugated steel decking is recommended to cover the plenum and support the slab above it. A small quantity of steel decking, however, was going to be too expensive to have shipped in, so we decided to use a double layer of corrugated galvanized-steel roofing. To make up for the weaker metal cover, we wired a grid of #4 rebar and positioned it over the plenum when we poured the slab. We cut one hole in the corrugated metal at each plenum for an inlet and an outlet to connect to the air ducts. Over each hole we formed a blockout with 2x redwood to create the inlet and outlet holes in the concrete and secured them by nailing the steel roofing to the bottom of the blockout. We covered the blockout with scrap 2x material to keep concrete out of the plenum during the pour, and held the top of the 2x cover flush with the top of the slab so that it would not be in the way when the concrete was finished. [t was later replaced by a double layer of :j.:ꅯ-in. plywood, which made a mounting plate for the fan. We cut the plywood to fit into the square hole left in the concrete when the temporary cover was removed and then cut a hole in the middle of the plywood to match the outlet hole of the fan. On this job, the ductwork and the registers above the slab were exceptionally simple to hook up. The intake register was located at the west end of the sunroom near the ceiling. [t was directly above the fan, so it required only a short piece of insulated flex duct to connect it with the fan. The register for the air returning to the sunroom from the block bed was located at the east end of the sunroom near the floor. This means that the cool air returning from the bedroom is circulated over the quarry-tile floor of the sunroom, where it can be warmed again. It then rises and is drawn into the intake register to complete the cycle. The only thing left to do was to wire the fan Laying an AlrCore blockbed. After the earth fill has been compacted, a poly vapor barrier is put down. Next, a 3-ln. to 4-ln. layer of sand Is screeded level to make a smooth base for the blockbed (top photo). This Is followed by 2 In. of extruded polystyrene foam-board Insulation. The blocks, with their cores aligned, are laid directly on top of the insulation (middle photo). The completed blockbed (bottom photo) will be covered with a 4-in. concrete slab. A sheet-metal cover will be placed over the plenums (seen in the foreground) to keep the concrete out. Grading and insulation for the sunroom floor can be seen in the background. 54 Fine Homebuilding and controls and wait for the sun to come out. Our solar hypocaust works quietly thanks to all the steps we took to muffle the fan noise. It also manages to keep the sunroom from overheating, and the bedroom stays warm and comfortable. The nice thing is that it does it all without taking up any room in the addition. 0 Photos: Bill Phelps